The principle of the silver salt diffusion transfer process is described in British Patent No. 614,155 filed Nov. 2, 1939. This process comprises the steps of exposing a photosensitive element containing a silver halide emulsion layer, developing the exposed photosensitive silver halide emulsion layer and forming a soluble silver complex of unexposed silver halide by treating the said photosensitive silver halide emulsion layer with an alkaline processing fluid in the presence of a developing agent and a silver halide complexing agent, transferring said soluble silver complex by diffusion to the silver receptive layer of an image-receiving element in superposed relationship with said silver halide emulsion, forming at said silver receptive layer an image incorporating silver from said silver complex under the action of development nuclei, and separating said image-receiving element from said photosensitive element. As explained in British Patent No. 614,155, the development nuclei are uniformly distributed throughout the silver receptive layer of the image-receiving element.
A more detailed description of the silver salt diffusion transfer process is provided in Chapter 16 (Author: G.I.P. Levenson) of "The Theory of the Photographic Process", Ed. T. H. James, 4th Edition, Macmillan, N.Y.
The silver salt diffusion transfer process has long been used in the field of graphic arts. In most applications in this field, images of high contrast are required, but there is also a need to produce continuous tone images by use of the-same processing conditions used for the high contrast images.
One method of reducing the contrast of a photographic silver halide layer is by using silver halide grains of different photographic speed. These grains can be blended and coated in a single layer or two separate layers can be coated, comprising a faster layer and a slower layer. This approach is taken in the materials described in European Specification 0 187 879.
U.S. Pat. No. 4,873,181 describes a photographic material comprising a support, a light-sensitive silver halide emulsion layer that contains silver iodide overlying the support, and an auxiliary layer containing non-light-sensitive silver halide grains having an average grain size of not more than 0.5 .mu.m. The purpose of the auxiliary layer is to provide high covering power, improved graininess and improved development stability.